Project Summary Adverse birth outcomes continue to be a major public health concern in the United States with outcomes such as low birth weight increasing over the past quarter century. However, known risk factors have been unable to account for all variability in adverse birth outcome risk and therefore research has expanded into examining environmental exposures such as metals as a source of risk. Exposure to inorganic arsenic has been of particular interest because nearly 13 million people in the US are exposed to levels higher than the maximum contaminant level set by the Environmental Protection Agency and exposure in pregnant mothers can cross the placenta and expose the developing fetus. It has been suggested in epidemiologic and experimental studies that maternal exposure to inorganic arsenic during pregnancy could be a critical exposure period for a developing fetus. Exposure has been associated with an increased risk for adverse birth outcomes including low birth weight, congenital heart defects, and fetal and infant mortality. Additionally, exposure to inorganic arsenic has been associated with increased risk for gestational diabetes mellitus and pregnancy-induced hypertension which are also risk factors for the above adverse birth outcomes. We propose to conduct a cohort study investigating the association between maternal exposure to inorganic arsenic in drinking water and the risk for adverse birth outcomes in the San Luis Valley, Colorado, a community with low to moderate (< 100 g/L) levels. The proposed study will address gaps in understanding of the embryotoxicity and etiology of inorganic arsenic on adverse birth outcomes and can aid in preventive efforts to limit arsenic exposure since primary exposure sources are well recognized.